1. Field of the Invention
The present invention relates generally to methods and apparatus for reducing harmonic distortion, and more particularly but not by way of limitation, to a harmonic distortion reduction system for providing at least a portion of a drive signal to a vibrator system adapted to impart energy into an object such as the earth wherein the portion of the drive signal provided by the harmonic distortion reduction system minimizes the harmonic distortion being generated from the vibrator system.
2. Brief Description of the Related Art
Vibrator systems such as servo-hydraulic vibrator systems impart energy waves into the earth at frequencies and magnitudes dictated by control electronics. The energy waves imparted into the earth by the vibrator system are used to determine the nature, orientation and location of subsurface formations. Generally, the energy waves created by the vibrator system travel through the earth's crust and are reflected back to the surface by various subsurface formations. These reflected energy waves are detected by means of seismic receivers.
Many sources of noise exist during data acquisition and it is therefore important to achieve the highest signal-to-noise ratio possible to enhance the detection of subsurface formations. For example, the vibrator system outputs not only energy waves of desired fundamental frequencies, but also of undesirable harmonic frequencies. These energy waves of undesirable harmonic frequencies are reflected onto the seismic records as noise. Merely increasing the energy output from the vibrator system to increase the energy waves of desired fundamental frequencies also increases the undesirable harmonic energy as well. Thus, increasing the energy output from the vibrator decreases the signal-to-noise ratio and increases the problems associated with harmonic energy. Existing control systems monitor the harmonic distortion level and reduce the vibrator output until the distortion level is below a predetermined threshold.
If one were to eliminate the noise associated with the undesirable harmonic frequencies, the energy output from the vibrator system could be increased which would improve the signal-to-noise ratio of the vibrator system. This would permit greater resolution in the seismic records which would thereby enhance the detection of subsurface formations, such as natural gas pockets. It is to such an improved control system having a harmonic distortion reduction system for providing at least a portion of a drive signal to the vibrator system to which the present invention is directed.